THE SENSES. 609 



peared likely to offer some explanation of the matter, especially as it was 

 also found that the pigmentation disappeared when the retina was ex- 

 posed to light, and re-appeared when the light was removed, and also 

 that it underwent distinct changes of color when other than white light 

 was used. The visual purple cannot, however, be absolutely essential 

 to the due production of visual sensations, as it is absent from the 

 retinal cones, and from the macula lutea and fovea centralis of the 

 human retina, and does not appear to exist at all in the retinas of some 

 animals, e. g. t bat, dove, and hen, which are, nevertheless, possessed of 

 good vision. 



If the operation be performed quickly enough, the image of an ob- 

 ject may be fixed in the pigment on the retina by soaking the retina of 

 an animal, which has been killed in the dark, in alum solution. 



Electrical Currents. According to the careful researches of 

 Dewar and McKendrick, and of Holmgren, it appears that the stimulus 

 of light is able to produce a variation of the natural electrical cur- 

 rent of the retina. The current is at first increased and then dimin- 

 ished. McKendrick believes that this is the electrical expression of 

 those chemical changes in the retina of which we have already spoken. 



VISUAL PERCEPTIONS AND JUDGMENTS. 



Reversion of the Image. The direction given to the rays by 

 their reflection is regulated by that of the central ray, or axis of the 

 cone, towards which the rays are bent. The image of any point of an 



FIG. 411. Diagram of the formation of the image on the retina. 



object is, therefore, as a rule (the exceptions to which need not here be 

 stated), always formed in a line identical with the axis of the cone of 

 light, as in the line of B a, or A b (Fig. 411), so that the spot where the 

 image of any point will be formed upon the retina may be determined 

 by prolonging the central ray of the cone of light, or that ray which 

 traverses the centre of the pupil. Thus A b is the axis or central ray of 

 the cone of light issuing from A; B a the central ray of the cone of light 

 issuing from B; the image of A is formed at b, the image of B at a, in 

 the inverted position; therefore what in the object was above is in the 

 image below, and vice versd the right-hand part of the object is in the 

 image to the left, the left-nand to the right. If an opening be made in 

 39 



